Automotive air conditioning systems generally comprise a refrigerant compressor, a condenser for removing excess heat from high pressure refrigerant, an expansion orifice for rapidly changing the pressure and temperature of the refrigerant, an evaporator for heat transfer between the air conditioning system and the passenger compartment of the automobile, and an accumulator for separating the gaseous and liquid refrigerant at the outlet of the evaporator. A suction line returns the gaseous refrigerant from the accumulator to the compressor. A small amount of oil is suspended in the refrigerant for lubrication of the moving parts of the compressor. The refrigerant level or charge therefore determines the degree of compressor lubrication as well as the cooling performance of the system.
If a significant portion of the refrigerant escapes, compressor lubrication may be insufficient, and continued operation under such conditions could severely damage the compressor. Accordingly, it has been proposed in certain prior art systems to sense the level of refrigerant and to disable the compressor if the refrigerant level falls below a reference level. Such systems, however, are usually expensive to implement since they require a special transducer for sensing the refrigerant level.
Other proposals have recognized that the compressor operation changes when the refrigerant level falls to a low level and the operation parameters are measured to detect the low level event. U.S. Pat. No. 4,463,573 to Zeno et al teaches using a pressure switch in the accumulator of an air conditioner and measuring the frequency of switch operation, a frequency higher than normal reflecting low refrigerant level. At a given frequency a warning signal is given and the compressor is disabled.
U.S. Pat. No. 4,463,576 to Burnett et al provides another arrangement for sensing the effect of low refrigerant on compressor operation. There, the duration of compressor energization periods is detected as an indication of low refrigerant charge. When the charge is low the compressor on time is short. To guard against false signals, three or more consecutive short on times is regarded as a valid low charge indication. If a condition of low refrigerant charge is detected, the compressor is disabled and an instrument panel lamp is lit. The present invention incorporates the teachings of Burnett et al, U.S. Pat. No. 4,463,576, which is incorporated herein by reference, and seeks to improve on the method of operation by adding still another indicator of low refrigerant charge.